A Research On Lithofacies Of Lacustrine Shale In The 4^th Submember Of The Third Member Of Shahejie Formation In Shichang Sub-sag,Nanpu Depression

In unconventional oil and gas exploration and development,shale oil resources account for a very important part.Lithofacies research is the basic work of shale oil exploration,due to the limited distribution of the continental lacustrine basin an the fast variations of lithofacies,it is difficult to conduct the lithofacies research.The shale formation of Es₃⁴ in Shichang-subsag is thick and broad,which has better hydrocarbon generation potential.However,studies on shale formation lithofacies are extremely less.Based on the previous research on sedimentary facies and distribution characteristics,the study first used wavelet analysis and frequency spectrum analysis to divide the sedimentary cycles based on logging data and core data,and established an isochronous stratigraphic framework.Total organic carbon testing,X-RD,thin-section identification and other methods were used to study the petrological and geochemical characteristics and divide the lithofacies.Then the lithofacies were comprehensively identified by fitting logging data with TOC value,mineral content and other parameters.Finally,the heterogeneity of lithofacies and the main controlling factors of lithofacies were studied under the stratigraphic framework.The study showed that Es₃⁴ can be divided into one long-term stratigraphic cycle,four medium-term stratigraphic cycles and ten short-term stratigraphic cycles.According to the classification criteria of"organic facies+component facies+structural facies",the lithofacies can be divided into 3 major-types,10 sub-types and 18minor-types.Through calculating each index of lithofacies by cross plots method,multiple linear regression method,?log R method,etc,the logging method can be used to identify lithofacies comprehensively,which can be more accurately.Lithofacies have different combinations in different cycles.Lithofacies have strong heterogeneity,which are reflected in the TOC content,carbonate minerals content,felsic minerals content and sedimentary structure.The TOC content shows a slight upward to downward trend from the bottom up.Carbonate minerals and felsic minerals content have a wide range of variation.The sedimentary structure of the shale strata is is mainly massive while the layered structure and lamellar structure are concentratedly developed in SSC6 and SSC8cycles.The main controlling factors of lithofacies are complex,organic facies are mainly affected by paleoclimate,paleoproductivity and the paleo-redox environment;component facies are mainly affected by tectonic movement,sedimentary source recharge,paleo-water depth and paleo-productivity;structural facies are mainly affected by paleo-salinity and water layering.